Piston



R.M.RODGER May 9, 1950 PISTON Filed April 15, 1946 i v Al HVVENTUR.

Patented May 9, 1950 UNITED STATES PATENT OFFICE Chrysler Corporation, Highland Park, Mich, a corporation of Delaware Application April 15,

10 Claims.

This invention relates to pistons for internal combustion engines and more particularly to pistons composed of a material having a greater coefllcient of thermal expansion than the material forming the cylinder walls in which the pistons are to operate.

The principal object of this invention is to provide a piston having a relatively great coefllcient of thermal expansion which is adapted to maintain a substantially uniform clearance between the thrust faces of the piston skirt and the confining cylinder walls.

It is also an object of this invention to provide a piston skirt which, although it has a small initial clearance from the cylinder walls, may attain a high engine operating temperature without binding on the cylinder walls.

The novel construction of my piston utilizes a proper relation between an elliptical contour, a particular distribution of varied skirt wall thickness, head and skirt separating slots and a particular distribution. of the mass of material in the structure to effect the desired results. The peripheral skirt contour of the piston when cold is elliptical. The peculiar construction of the piston controls the transmission of heat and expansion forces so that the peripheral skirt contour when heated is relatively circular, and the minimum clearance between skirt and cylinder wall at the skirt thrust faces is substantially constant.

In the drawings which illustrates a preferred embodiment of my invention,

Fig. 1 is a vertical side elevation of my piston shown in sectioned cylinder walls.

Fig. 2 is a section on the line 2-4 of Fig. 1.

Fig. 3 is a transverse section of the piston on a plane normal to the axis of the wrist pin bosses.

Fig. 4 is a section on the line l-4 of Fig. 2, but with the cylinder walls omitted therefrom.

Fig. 5 is a section on the line 5-5 of Fig. 2 with the cylinder walls omitted therefrom.

Referring to the drawings, the cylinder walls ill which are usually made of cast iron surround the piston ii commonly formed from an aluminum alloy. The piston ll comprises a head I: and a skirt ii. The head comprises a disc from which depends a ring flange It provided with ring grooves ii for receiving rings it. The skirt I! forming a continuous substantially cylindrical shell is secured to the head I2 and provided with wrist pin bosses II formed therein. The bosses are integral with and extend from the wall of the skirt. Adjacent the bosses H the head It and skirt II are integrally united. A web ll 1946, Serial N0. 662,122

(on. soc-11) also materially increases the thickness of the metal in the head in the plane of the wrist pin bosses.

The skirt I3 is separated from flange H by air gaps or horizontal slots l8 throughout a considerable portion of their respective adJacent surfaces. The norizontai slots I! are in diametrically opposite sides of the skirt and are symmetrical with respect to a vertical plane normal to the axis of the bosses. The length of the slots may vary, but in the preferred construction they are as long as possible. The factor which limits the length of the slots is the strength of the connection between the head and the skirt adjacent the wrist pin bosses. It is desired that the length of the slots be limited only by the strength requirement of this connection.

The longitudinal sectional contour of the outer surface of the skirt is substantially uniform 20 throughout its length except for the usual relleved surface 20 adjacent the wrist pin bearing surfaces 2|. However, the outer peripheral surface of the skirt varies from a true cylinder as illustrated in Figs. 4 and 5. A dotted reference circle 22 is illustrated therein. In a horizontal section the contour of the outer perimeter is substantially elliptical. Diametrically opposite areas 23 adjacent the wrist pin bosses are relieved below the true circular contour of the reference circle 22, usually by grinding. It is preferred that the relief be gradual and at its maximum depth in the planeof the wrist pin axis and at its minimum depth in a plane normal to the wrist pin axis. The diametrically opposite surfaces 24 in the plane normal to the wrist pin axis are on the major diameter of the skirt and form the thrust faces of the piston which take the lateral pressure of the piston caused by the angularity of the connecting rod to the piston. The elliptical contour of the piston makes surfaces 24 a substantially straight line contact with the cylinder wall when the piston is cold.

The thickness of the skirt wall is symmetrical but not uniform. The skirt wall decreases in thickness from the plane of the wrist pin axis to its minimum thickness in a plane normal to this axis. This variation in thickness is illustrated in an exaggerated degree in Figs. 4 and 5.

In the operation of this piston when it is cold the closest fit of the skirt with the cylinder wall occurs at the thrust surfaces 24. I have rovided means to restrict the thermal expansion of the piston skirt in the plane of surfaces 2i 7 so that the piston does not bind on the cylinder wall at these surfaces. Web It, bosses ll. the

aooaai 1 variation in thickness of the depending ring flange ll caused by the presence of web II and the thickness of the skirt in areas 23 provide the greater mass of metal in the plane of the axis of the bosses as contrasted to the plane normal to this axis. In this latter plane the skirt has its thinnest section at 24 and the head I2 is thin due to the absence of web IB. Note the difference in thickness of flange II in Figs. 2 and 3. The major portion of the heat created in the head 12 as an incident to engine operation is transmitted, therefore, by the relatively great mass of metal to that portion 23 of the skirt in the plane of the axis of the bosses. The slots is accentuate this concentration of heat transfer by actually separating the head II from the thinner areas 2 of the skirt. The concentration of heat in that portion 23 of the skirt in the axis of the bosses causes the skirt to expand more in this plane than in a plane normal thereto. Figs. 4 and 5 illustrate that at the former plane the skirt has relieved areas 23 which provide the space in which the expansion can occur.

The tendency oi the skirt to expand in the direction of the axis of the bosses because of the transmission of heat in that direction is accentuated by the simple fact that when the head which is the hottest portion of the piston exparids radially in all directions in the plane of its upper surface this motion is actually transmitted to the skirt only at the connections between the head and skirt adjacent the wrist pin bosses. The slots l9 interrupt the transmission of this motion on all radii included in the arcs described by the slots.

The expansion of the skirt in the direction of the plane of the axis of the bosses induces lateral forces tending to pull areas 24 radially inward. Therefore, the skirt diameter actually increases in the plane of surfaces 23 and the lateral forces from this expansion tend to equalize or overcome any thermal expansion on the diameter normal to the axis of the bosses.

To cooperate with the above factors in assuring that the thrust surfaces 24 will not bind in the cylinder walls as a result of expansion, the relatively thin skirt walls are provided at II. The tapered thickness of the skirt wall from a maximum thickness adjacent the plane of the axis of the bosses to a minimum thickness in a plane normal to the first plane confines the inward radial deflection of the skirt wall incident to the radial skirt expansion in the plane of the boss axis to the thin skirt wall portions.

The piston described herein even when cold provides a substantially close fit between the skirt and the cylinder walls at the thrust surface of the skirt where it is needed. When the piston is hot, incident to engine operation, the expansion of the skirt is absorbed in the plane of the axis of the bosses where it is harmless because relieved surfaces 23 have been provided. The force on the skirt thrust surfaces which is the resultant of minor thermal expansion forces in an outward direction and oppositely directed forces caused by deflection of the thin walls is negligible and the major diameter of the ellipse is substantially constant or increases no more than the diameter of the cylinder itself due to the expansion of the heated engine block.

I claim:

1. A hollow piston for an internal combustion engine comprising a head having a depending ring flange and a skirt, said skirt being provided with a pair of diametrically opposed wrist pin bosses, the wall thickness of said skirt immediately above said bosses being symmetrical and varying from a maximum thickness in a first plane containing the axis of said piston and the axis of said wrist pin bosses to a minimum in a plane normal to said first plane and containing the axis of said piston, said minimum thickness in said last mentioned plane being thinner than all intermediate portions of said skirt wall, said flange and said skirt being integrally connected through a pair of diametrically opposed sections and separated by a pair of diametrically opposed air gaps throughout portions of their adj acent surfaces.

2. A hollow piston comprising a head and a skirt, the transverse cross section of said skirt presenting an exterior edge having an oval contour, said skirt adjacent said head having relatively thick wall sections adjacent its minor axis and relatively thin wall sections adjacent its major axis so proportioned that thermal distortion of said skirt will cause said edge to assume a circular contour.

3. A hollow piston for an internal combustion engine comprising a head having 9. depending ring flange and a skirt, said skirt being provided with a pair of diametrically opposed wrist pin bosses, the wall thickness of said skirt in a plane between said wrist pin bosses and said flange being symmetrical and varying from a maximum thickness in a first plane containing the axis of said piston and the axis of said wrist pin bosses to a minimum in a plane normal to said first plane and containing the axis of said piston, said flange and said skirt being integrally connected through a pair of diametrically opposed sections and separated by a pair of diametrically opposed air gaps throughout portions of their adjacent'surfaces, said skirt having a elliptical exterior contour with its minor axis in said first plane and its major axis in said normal plane, said thicknesses and said air gaps being so proportioned that thermal distortion of said skirt will produce a circular exterior contour by increasing the outside diameter in said first plane and decreasing the outside diameter in said normal plane.

4. A piston comprising a head and a skirt, said skirt being provided with a pair of diametrically opposite wrist pin bosses, said skirt having an elliptical transverse section and thrust surfaces symmetrical with respect to the major axis of the skirt above said wrist pin bosses, the thickness of the skirt being symmetrical and the minimum thickness being localized on the major axis of the skirt and the maximum thickness being adjacent the minor axis of the skirt, said localized minimum thickness accommodating radially inward distortion of said skirt on said major axis as an incident to radial expansion of said skirt on said minor axis in response to heating of said piston.

5. A hollow piston comprising a head and a skirt, said skirt being substantially elliptical in transverse cross section, said skirt having relatively thick wall sections adjacent its minor axis and relatively thin wall sections adjacent its major axis, said thin wall sections being separated from said head by a pair of transverse slots symmetrical to the major axis of the skirt and being integrally united with said thick wall sections at the outer edges thereof.

6. A piston comprising a head and a skirt, said skirt being provided with a pair of diametrically opposite wrist pin bosses, said skirt having an el- 6 llptical section and thrust suriaces symmetrical with respect to the major axis of the skirt, the thickness of the skirt being symmetrical and the minimum thickness being local bed on the major axis oi the skirt between said bosses and said head and the maximum thickness being adiacent the minor axis of the skirt, said skirt being separated from said head by diametricaliy opposite openings adjacent the major axis of the skirt, the portions of said skirt carrying said thrust surfaces being secured to the outermost portions of said bosses so that thermal expansion oi said bosses will induce a maximum contraction oi said portions along said major axis.

7. An internal combustion engine piston adapted to reciprocate within an engine cylinder including a head and skirt integrally formed of light metal alloy, said skirt including opposed thrust faces separated at their upper edges from the head and supported at their sides fromthe head, aligned wrist pin bosses disposed on opposite sides of said thrust i'aces, said skirt having an oval cross sectional contour with the major axis perpendicular to the axis of said wrist pin bosses and proportioned so as to be flexed into coniormity with the engine cylinder wall by thermal expansion.

8. An internal combustion engine piston including a head and skirt integrally formed, said skirt including opposed thrust faces and aligned wrist pin bosses disposed on opposite sides of said thrust faces, the thickness 01' said skirt in planes below said wrist pin bosses gradually varying from a. maximum in the axial plane 01' said bosses to a minimum in a second axial plane at right angles to the first plane.

9. A piston comprising a head and a skirt integrally connected to said head by a pair oi diametrically opposite connectors having relatively short circumferential length, said skirt having a wall thickness which gradually decreases from a maximum in a first longitudinal plane intersecting said connectors to a minimum wall thickness in a longitudinal plane normal to said first plane.

10. A piston comprising a head and a skirt, a pair of diametrically opposite wrist pin bosses carried by said skirt, a rib extending across the entire width of said head and merging with said bosses, said skirt having relatively thick sections in a first plane containing said rib end sections of gradually decreasing thickness between said thick sections.

ROBERT M. RODGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,494,483 Howe May 20, 1924 1,774,277 Kirby Aug. 26, 1930 2,136,822 Moore Nov. 16, 1938 2,221,535 Berry Nov. 12, 1940 

